First, preheat your oven to 350°F (175°C) so it’s ready for toasting the sandwiches. In a large skillet over medium-high heat, melt the butter. Add the thinly sliced onions and cook them for about 4-5 minutes, stirring occasionally until they soften and start to brown.
Add the thinly sliced chicken to the skillet with the onions. Season generously with salt and pepper. Continue to cook, stirring frequently, for another 4-5 minutes, or until the chicken is fully cooked through and no longer pink.
Once the chicken is cooked, pour the buffalo sauce into the skillet. Stir everything together until the chicken and onions are evenly coated in the sauce. Let it simmer for a minute to allow the flavors to meld together.
Slice your hoagie rolls open, but not all the way through. Place two slices of provolone cheese on each roll, then pile the buffalo chicken mixture on top of the cheese. Wrap each sandwich tightly in aluminum foil and place them on a baking sheet. Bake for 5 minutes, just until the bread is warm and the cheese is perfectly melted and gooey.